420 FUNCTIONS OF NERVES. 



sign of sensibility appears on cutting the former to a great depth, 

 or the latter superficially. But the division of the trigeminum, 

 on the sides of the fourth ventricle, has all the effects of its di- 

 vision without, and severe pain attends injury of the interior and 

 sides of the fourth ventricle 1 , except as you approach the anterior 

 part of the spinal chord ; and there is little sensibility at the cor- 

 pora quadrigemina. The effects of the division of the spinal chord 

 are of course more extensive in proportion as the division is made 

 higher up ; and, if made above the origin of the phrenic nerves, 

 which are the chief agents in causing the contraction of the in- 

 spiratory muscles, and consequently above the origin of all the 

 nerves of inspiration, death immediately ensues. Yet, in brutes, 

 after removing the head or dividing the spinal chord, if any 

 limb is irritated, its muscles are thrown into action : thus Sir 

 Gilbert Blane, after such operations in kittens a few days old, 

 found the hind legs to shrink from the touch of a hot wire applied 

 to the hind paws ; and the tail to move when irritated, after the 

 division of the chord below the last lumbar vertebra." More 

 divisions than one do not prevent this effect. If the head of a 

 pigeon is cut off, and the whole brain removed except a portion 

 to which the third pair is attached, and the optic nerve is divided, 

 the iris instantly contracts when the extremity of the optic nerve 

 is pinched. Dr. Macartney says that contraction of the iris 

 occurs from light suddenly admitted to the retina after the head 

 is cut off or the eye taken out.? Dr. Magendie also remarks 

 that, when the posterior roots of the spinal chord are irritated, 

 besides signs of extreme pain, the muscles below the part irritated 

 are thrown into action, but only on the same side of the body. 

 All these facts show a peculiar relation between the nerves of 



i Dr. Magendie, Freds, t.i. p. 237. 3d edit. 



m It is thus that animals are every day killed by pithing ; in Germany I have 

 never seen oxen killed in any other way : a blow on the back of the neck is 

 sufficient to destroy rabbits. Livy informs us that, at the suggestion of As- 

 drubal, in the battle in which he was slain, when the Carthaginian forces were 

 routed, and their elephants became unmanageable, the drivers destroyed them 

 in a moment by one blow of a hammer upon a knife fixed between the junction 

 of the head and spine. (Histor. 1. xxvii. c. 49.) The division of the phrenic nerve 

 only does not put a stop to respiration. See for instance, Bichat, Recherches 

 Physiol. p. 327. 



n Select Dissertations on several Subjects of Medical Science. By Sir Gilbert 

 Blane, Bart. M.D. London, 1822. p. 262. 



Mr. Mayo, 1. c. p. 231. 



p Report of the Third Meeting of the British Association, 1834, p. 53. 



